The ability of an organism to regulate the expression of its genes is of central importance to life. A breakdown in this homeostasis leads to disease states, such as cancer, where a cell multiplies uncontrollably, to the detriment of the organism. The general mechanisms utilized by organisms to maintain this gene expression homeostasis are the focus of intense scientific study.
It recently has been discovered that some cells are able to down-regulate their gene expression through certain ribonucleic acid (RNA) molecules. Namely, RNA molecules can act as potent gene expression regulators either by inducing mRNA degradation or by inhibiting translation; this activity is summarily referred to as post-transcriptional gene silencing or PTGS for short. An alternative name by which it is also known is RNA interference, or RNAi. PTGS/RNAi has been found to function as a mediator of resistance to endogenous and exogenous pathogenic nucleic acids and also as a regulator of the expression of genes inside cells.
The term ‘gene expression,’ as used herein, refers generally to the transcription of messenger-RNA (mRNA) from a gene, and its subsequent translation into a functional protein. One class of RNA molecules involved in gene expression regulation comprises microRNAs, which are endogenously encoded and regulate gene expression by either disrupting the translation processes or by degrading mRNA transcripts, e.g., inducing post-transcriptional repression of one or more target sequences.
The RNAi/PTGS mechanism allows an organism to employ short RNA sequences to either degrade or disrupt translation of complementary mRNA transcripts. Early studies suggested only a limited role for RNAi, that of a defense mechanism against pathogens. However, the subsequent discovery of many endogenously-encoded microRNAs pointed towards the possibility of this being a more general, in nature, control mechanism. Recent evidence has led the community to hypothesize that a wider spectrum of biological processes are affected by RNAi, thus extending the range of this presumed control layer.
A better understanding of the mechanism of the RNA interference process would benefit drug design, the fight against disease, and the understanding of host defense mechanisms.